1. Field of the Invention
The present invention relates to a formed product of an L-lactic acid base polymer and the preparation process of the product. More particularly, the invention relates to a formed product of an L-lactic acid base polymer which has good transparency and can degrade after use in natural environment.
2. Related Art of the Invention
Plastic materials for forming products prepared from plastic sheets are polystyrene, polyvinyl chloride, polypropylene and polyethylene terephthalate. Although some of the formed products prepared from these plastics are excellent in transparency, disposal of the products increases the amount of refuse and they remain semipermanently when buried under the ground, since they almost do not degrade under the natural environment. Disposal of the formed products in the ocean causes aesthetic damage or destruction of the living environment of marine organisms.
On the other hand, polylactic acid and copolymers of lactic acid and hydroxycarboxylic acids other than lactic acid (generic name of the polylactic acid and the copolymers will hereinafter be referred to simply as a lactic acid base polymer) have been developed for thermoplastic and biodegradable polymers. These polymers can be completely biodegraded within a few months to an year in an animal body. When the polymers are placed in most soil or sea water, they start to decompose in a few weeks in a moist environment and disappear within a year to several years. Further, final degradation products of the polymers are lactic acid, carbon dioxide and water which are nontoxic to human body.
Polylactic acid in the lactic acid base polymer is usually prepared from a cyclic dimer of lactic acid which is called lactide, and U.S. Pat. Nos. 1,995,970, 2,362,511, and 2,683,136 have disclosed a polymerization process of lactide. U.S. Pat. Nos. 3,636,956 and 3,797,499 have disclosed a process for copolymerizing lactic acid and glycolic acid. In the copolymerization of lactic acid and the other hydroxy carboxylic acids, lactide and a cyclic ester intermediate, for example glycolide, i.e., a dimer of glycolic acid, are used for polymerization.
Lactic acid which is a raw material of the lactic acid base polymer is prepared by fermentation or chemical syntheses. Recently, L-lactic acid has been cheaply produced in a large amount by fermentation in particular. The polymer obtained has the characteristic of high stiffness. Consequently, the polymer having a high content of L-lactic acid is now under development for use.
Particularly, formed products obtained by preparing a sheet from a polymer having a high content of L-lactic acid and subjecting the sheet thus prepared to vacuum forming or air-pressure forming are expected to have a bright future because forming can be conducted with ease and many kinds of desired shapes can be formed. However, the formed products prepared from the polymer having a high content of L-lactic acid through the above forming method are opaque and are restricted in the fields of use. The opacity is particularly enhanced when the L-lactic acid proportion is 75% or more in the lactic acid base polymer. The phenomenon is thought to be inhibition of transparency by the crystallized lactic component because poly-L-lactic acid is very liable to crystallize in the polymer.